The sudden deaths of tens of thousands of endangered antelopes in Kazakhstan over the past two weeks have left scientists scrambling for answers and conservationists worried about the animal's future.

More than 120,000 rare saiga antelopes — more than a third of the total global population — have been wiped out in a devastating blow that the United Nations Environment Programme has called "catastrophic".

Scientists are struggling to put their finger on the exact nature of the disease that has felled entire herds, but say findings point towards an infectious disease caused by various bacteria.

Kazakhstan's prime minister Karim Massimov has set up a working group including international experts to establish reasons for the deaths and oversee disinfection of lands in the three regions where the saiga died.

UN experts said the mass deaths come down to "a combination of biological and environmental factors".

Any infections have likely been exacerbated by recent rains that have made the antelopes less able to cope with diseases.

"Unseasonal wetness may have been something that lowered their immunity to infection but until we do more analysis we will not know anything for sure," Steffen Zuther of the Altyn Dala Conservation Initiative said.

The rate of the deaths has staggered those who have studied the species.

"A 100 per cent mortality for the herds affected is extraordinary," professor Richard Kock from the Royal Veterinary College in London said.

"We are dealing with creatures that have fairly low resilience."

Numbers could take a decade to recover

The sudden spate of deaths has come as an upset to conservationists, who had hailed the prosperity of saiga antelopes herds as a conservation success.

Before the first reports of mass deaths in mid-May, saiga numbers in Kazakhstan had exploded from an estimated 20,000 in 2003 to the more than 250,000.

In 1993, there were more than 1 million saiga antelopes, mostly concentrated in the steppe land of Kazakhstan, neighbouring Russia and Mongolia.

The susceptibility of the population since then has raised extinction fears and the saiga is listed as critically endangered by the International Union for Conservation of Nature (IUCN).

Scientists have estimated that it will take a decade for the antelope numbers to recover from the recent deaths.

Herds that have not yet been struck down are thought to be safe for the moment, but scientists are hoping the beasts can avoid the threat of even more potent diseases, such as the morbillivirus, an epidemic of which swept across neighbouring China last year.